1. Field of the Invention
The present invention relates to a process for refining a feed oil of petroleum type to produce a lubricating oil. More particularly, the present invention relates to a process for refining a feed oil to produce a lubricating oil, wherein the switch between feed oils can be conducted in a short time.
2. Description of the Related Art
In refining a feed oil of petroleum type to produce a lubricating oil, an equipment for lubricating oil production is generally used which comprises a furfural refining unit (hereinafter referred to as FRU in some cases), a hydrotreating unit (hereinafter referred to as HTU in some cases) and a ketone dewaxing unit (hereinafter referred to as KDU in some cases). FRU has at least an extraction column, a heater fixed to the extraction column and a rotating disc contactor provided in the extraction column, and is for refining a feed oil using a furfural type solvent. HTU has at least a reactor and is for reacting a feed oil supplied from FRU, with hydrogen in the presence of a catalyst. KDU has at least a plurality of filters and is for adding a ketone type solvent (e.g. mixture of methyl ethyl ketone or methyl isobutyl ketone or acetone and benzene or toluene) to a feed oil supplied from HTU, at least two times (primary addition and secondary addition) to cool the feed oil and then subjecting the cooled feed oil to filtration to remove the precipitate (mainly wax) present in the cooled feed oil. These units are connected in the order of FRU, HTU and KDU, and the product oil of one unit is used as a feed oil for the next unit. It is generally preferable that no intermediate storage tank be provided between two adjacent units.
A process for refining a feed oil to produce a lubricating oil by using such an equipment for lubricating oil production is described. First, in FRU, non-paraffinic components (e.g. resinous component) are removed from a feed oil by the use of a furfural type solvent, and the components not dissolved in the solvent are sent to a next unit (HTU) as a product oil having an improved viscosity index. The furfural type solvent is removed in a solvent recovery system and circulated for reuse. Then, in HTU, the product oil of FRU is reacted with hydrogen on a catalyst under the conditions of high temperature and high pressure to remove the impurities (e.g. sulfur compounds) present in the product oil. The resulting oil is treated in a stripper of HTU to remove the gas components (e.g. hydrogen). The refined oil supplied from HTU is sent to KDU and is mixed with a ketone type solvent consisting of, for example, methyl ethyl ketone and benzene or toluene. The mixture is cooled under given conditions and the wax components precipitated as crystals in the mixture are removed by filtration. By the above operation can be obtained a final product (a lubricating oil) having improved low-temperature properties including improved fluidity.
The feed oil used in the above equipment, i.e. the feed oil supplied to FRU is produced from a crude oil via various steps of atmospheric distillation, vacuum distillation, propane deasphalting, etc., is stored in a feed oil tank provided upstream of the equipment for lubricating oil production, and is supplied from the tank. In this case, a plurality of feed oils are prepared depending upon the type of the crude oil used and the desired properties of the lubricating oil to be produced, and a desired lubricating oil is produced by selecting an appropriate feed oil.
In ordinary process plants including the above equipment for lubricating oil production, the control system for operation aims at control of operation in steady state. In the above equipment for lubricating oil production, it is desired to maintain the viscosity, viscosity index and pour point of a lubricating oil to be produced as a final product, at given levels. Hence, the operation of said equipment is controlled so that the operation conditions necessary for treating a feed oil of certain properties to produce a lubricating oil of desired properties are maintained. Therefore, it is conducted, for example, to control, by feed back, the amount of the feed oil to be supplied to the heating furnace so that the outlet temperature of the heating furnace for feed oil is kept constant, or to control, by feed back, the operation conditions of the pump for feeding a solvent into the extraction column so that the flow rate of the solvent in the extraction column is kept constant.
In an equipment for lubricating oil production, such as mentioned above, when the feed oil is switched from one kind to another kind, the equipment is in a non-steady state during the switch period. Herein, the non-steady state refers to a state in which as a result of the switch of the kind of feed oil, i,e, the change of the properties of lubricating oil (final product), the viscosity, etc. of lubricating oil see transitional changes. The lubricating oil obtained during a non-steady state is neither the one corresponding to the feed oil before switch nor the one corresponding to the feed oil after switch, and is an off-specification product. Therefore, for the effective operation of the equipment for lubricating oil production, it is necessary to make the non-steady state as short as possible and reduce the amount of the off-specification product obtained during said state.
In the equipment for lubricating oil production, the control system for operation has conventionally aimed at control of operation in steady state. Hence, the time period of non-steady state operation cannot be made short (the total time required for the switch of feed oil is typically about 10 hours); a large amount of an off-specification lubricating oil is generated; and the procedure associated with the switch of feed oil is complicated, posing a big burden on operators.